A hydraulic system for a construction machine in the related art, as illustrated in FIG. 1, includes a variable displacement hydraulic pump (hereinafter referred to as a “hydraulic pump”) 1 connected to an engine (not illustrated); an attachment operation device 2 and a swing operation device 3 outputting operation signals in proportion to operator's operation amounts; at least one attachment actuator 5 connected to a discharge flow path 4 of the hydraulic pump 1 to be driven by the operation of the attachment operation device 2; a swing motor 7 connected to a flow path 6 branched from the discharge flow path 4 to be driven by the operation of the swing operation device 3; at least one first control valve 8 installed in the discharge flow path 4 and shifted to control a start, a stop, and a direction change of the actuator 5 in response to the operation of the attachment operation device 2; a second control valve 9 installed in a flow path 6 and shifted to control a start, a stop, and a direction change of the swing motor 7 in response to the operation of the swing operation device 3; check valves 20 and 21 respectively installed in flow paths a and b for detecting load pressures of the attachment actuator 5 and the swing motor 7 to take a maximum pressure (i.e., maximum load sensing pressure) of the detected load pressures; a first pressure compensation valve 10 installed on a downstream side of the discharge flow path 4 to have an opening amount that is controlled by a difference between the maximum load sensing pressure and the pressure on the downstream side of the discharge flow path 4; a second pressure compensation valve 11 installed on the downstream side of the flow path 6 to have an opening amount that is controlled by a difference between the maximum load sensing pressure and the pressure on the downstream side of the flow path 6; and a control unit 16 receiving an input of detection values of a detection means 14 for detecting the operation amount of the attachment operation device 2, a detection means 15 for detecting the operation amount of the swing operation device 3, a maximum load pressure sensor 13 detecting a maximum load sensing pressure; and a pressure sensor 12 for detecting a discharge pressure of the hydraulic pump 1, and outputting a control signal for controlling a discharge flow rate of the hydraulic pump 1 through a flow control valve 17.
In an excavator which is provided with a pressure compensated valve and to which a load sensing valve is applied as described above, if a difference between a hydraulic pump discharge pressure value that is detected by the pressure sensor 12 detecting the discharge pressure of the hydraulic pump 1 and the maximum load pressure value detected by the maximum load pressure sensor 13 detecting the maximum load sensing pressure is equal to or higher than a preset value, the flow control valve 17 may operate to reduce the discharge flow rate of the hydraulic pump 1. That is, if the above-described pressure difference is high, the control unit 16 operates to reduce the discharge flow rate of the hydraulic pump 1, while if the above-described pressure difference is low, the control unit 16 operates to increase the discharge flow rate of the hydraulic pump 1.
That is, if the load pressure of the attachment actuator 5 is low, the load pressure of the swing motor 7 becomes the maximum load sensing pressure, and the opening area of the first pressure compensation valve 10, which has the load pressure that is lower than the maximum load sensing pressure, is reduced through the first and second pressure compensation valves 10 and 11. Through this, pressure is formed to correspond to the maximum load sensing pressure of the attachment actuator 5 and the swing motor 7, and thus combined operability can be secured.
On the other hand, in the case of a combined operation in which the swing motor 7 is operated by the operation of the swing operation device 3 and the attachment operation device 2 is operated at the same time, the load pressure on the swing side relatively becomes very high. Due to this, a spool of the first pressure compensation valve 10 connected to the attachment is shifted to be almost closed, and thus the flow rate that is supplied to the attachment actuator 5 becomes low. Accordingly, the attachment, such as a boom, is hardly operated to lower the combined operability.